Add capacity for a factor covariate

[MNordborg]

No description provided.

[beckyfisher]

At this time bnec does not allow inclusion of an interaction with a fixed factor. Including an interaction term within each of the non-linear models implemented in bayesnec is relatively straightforward, and may be introduced in future releases. However, in many cases the functional form of the response may change with different levels of a given factor. The substantial complexity of defining all possible non-linear model combinations at each factor level means it unlikely this could be feasibly implemented in bayesnec in the short term. In the meantime the greatest flexibility in the functional form of individual model fits can be readily obtained using models fitted independently to data within each factor level. The compare_posterior function allows these individual models fits to be compared.